Iwao Nakajima

Effect of Internal Gas Pressure on Gas Permeability of Coal in Unsteady-State Gas Flow. Study on gas permeability of coal by crack network model (2nd Report).

The purpose of this study is to prove the effect of internal pore pressure on unsteady-state gas flow in coal. Practically, the gas permeation model of coal which consists of the gas permeable and non-permeable crack network was proposed. Then the numerical simulations of gas permeation by this model were carried out by changing the parameters such as inlet pressure in the crack network, confining pressure against coal, closing probability of cracks, etc.The results of these simulations on fluctuations of unsteady-state gas flow were in agreements with the experiments of gas permeation of coal. From these considerations it was found that the above gas permeation model was accurate enough for practically. Moreover, the results of the simulations on unsteady-state gas flow in coal gave an explanation to the deviation from the Darcys law in the relationsbetween gas flow and difference pressure.

Vegetation Limit and Erosion Limit of Artificial Soil. Development of artificial soil for vegetative slope(2nd Report).

In this study non-erosive soil materials have been developed in order to realize a green slope in opencut mining. Practically, the vegetation limit and the erosion limit of artificial soil were determined on the basis of the vegetation tests of lawn grass and the erosion tests by water jet respectively.The specimens of soil were made of the tailings of cyanide leaching at the Kushikino gold mine through the following two processes;(1) a mixture of the tailings, water, portland cement and woody fi ber was crushed under 12 mesh after the primary generation of cohesive strength for a specific time, (2) the crushed products were applied to the tests of the secondary generation of cohesive strength after charging in a pot and consolidating under various compressive stress.From the results of the above tests it was proved that the cohesive strength R{τ, ρ, η} of the soil could coexist within the range between the vegetation limit Vc and the erosion limit Ec as given by the following inequality.Vc >R{τ, ρ, η} >EcHere, the time τ of the primary generation of cohesive strength, the consolidating stress ρ and the fiber content η are the parameters of the soil strength. The measured values of Vc and Ec were 140 and 60 kg f/cm2 respectively.

Development of Non-erosive Soil Materials. Development of artificial soil for vegetative slope (1st Report).

In order to realize the vegetative slope in open-cut mining, non-erosive soil materials have been developed in this study.The non-erosive soil was compounded from the tailings of cyanide leaching at the Kushikino gold mine through the following two processes ;(1) a mixture of the tailings, water and portland cement was crushed under 12 mesh after the primary generation of cohesive strength for a specified time, (2) the crushed products were applied to the test of the secondary generation of cohesive strength after charging in a pot and consolidating under various compressive stresses.The structure of the non-erosive soil is composed of soil particles and voids surrounding the particles. The soil particles and voids are formed as the results of the primary generation of cohesive strength before the crushing and the secondary generation of cohesive strength after the consolidating respectively. The soil properties required to satisfy the vegetative conditions result from such agglomerate structure of the soil particles. From the facts described above it is concluded that the vegetative and the non-erosive conditions of the soil are satisfied simultaneously by controlling the curing and solidifying time from mixing to crushing and the secondary generation of cohesive strength.

Experimental study on internal fracturing and outburst phenomenon of porous material by pore gas pressure.

Generally, many tensile cracks could develope derivatively along a large fracture generated mechanically in the coal seam ahead of a working face when high pressure gas is confined in micro pores and micro cracks and gas is almost impermeable. Gas pressure in the fractured zone increases intensively owing to free gas from the micro pores and the micro cracks, when the chain reactive fracturing occurs in a coal seam. Generation of these fractured zones must be a cause of coal and gas outburst.In this research, scale model experiments were carried out by using the rosin including numerous micro pores saturated with high pressure gas in order to ascertain the occurrence of the phenomena above mentioned. As the result of the experiment, it was confirmed that both the tensile fracturing and gas outburst occurred as predicted.

Rock and Water Outburst and Its Prevention Measure.

A kind of rock and water outburst occurred several times at the deep level excavation of the shafts or the inclines in Yubari New Coal Mine in the 1970s. In this incident a large amount of groundwater containing dissolved gas flowed out with scaled rock and a crater was formed at the outburst trace.In this study, the formation mechanism of the above crater was considered on the basis of the observational and the experimental results in order to obtain a prevention measure against the outbu st. Moreover, the shape of the crater was examined by applying the probability theory on material strength to the bending fractures of the scaled rock.From these results, it became clear that the vertical angle of the crater depended on the coefficient of uniformity of the rock strength and the hydrostatic pressure gradient in the cover rock. And, the observational facts, which the vertical angle of the crater in sandstone was larger than that in shale, could be explained well. Consequently, it became possible to estimate the grouting depth at the prevention works site against the rock andwater outburst.

The application of an electric probe for measuring local concentrations of slurry.

An electric conductivity method was applied to the measurements of local concentrations of spherical particles in slurry.A probe, equipped with two ringlet electrodes on its surface, was utilized for measuring the resistivity of water and slurry.An AC source was used to supply current to the electrode circuit and this was effective in eliminating the accumulation of scales and bubbles which gave rise to instability on the electrode surface.Since the resistivity increment due to solids was expressed as the relative resistivity divided by that of water, the effects of water resistivity fluctuation and position relative to the pipe wall were eliminated.Probe calibration carried out both in fluidized tests and in flow tests through a vertical downward pipe showed dependence on particle size.The data obtained by the two methods indicated different tendencies; the fluidized tests were plotted as straight lines and the flow tests were plotted as curved lines on log-log representations.Since the discharge concentration calculated using the calibration curves from the fluidized tests showed lower values in the high concentration region, the calibration curves obtained from the flow tests were used for concentration measurement.The curves were well represented by a modified version of Maxwells equation including compensated terms for particle size and concentration.The accuracy of the calculated discharge concentration in a non-conductive horizontal pipe and in a conductive vertical pipe was 7.1% in the worst case.Although the application of this method was limited to particles with narrow size distributions, a consistent picture of solid concentration profiles could be presented.